For example, in a welding gun used for welding work, a common air cylinder is generally used. The welding gun is configured to perform welding operation while applying a required pressurizing force to work after having clamped the work by a clamping mechanism. However, when work is to be clamped, a welding tip may collide against the work and deforms it. This results in poor welding. With this being the situation, the welding gun is formed by an air servo cylinder, and in order to reduce the shock at a time when work is clamped and to shorten the welding time by decreasing pressurization time after the clamping, an air cushion mechanism is attached to the cylinder, as well as a changeover valve or a quick exhaust valve is provided in a drive circuit for the cylinder, whereby the air cylinder is allowed to perform proper operations.
However, the air cushion mechanism attached to the air cylinder can exert a cushioning action only at a specified position such as a stroke end. When work is clamped at a plurality of positions, that is, when the thickness of work varies and target positions are not invariant, a shock reduction by the air cushion cannot be achieved. Also, provision of an air cushion, a changeover valve, and a quick exhaust valve besides servo valves can raise problems of incurring a large size, heavy weight, high cost, short lifetime, low-reliability and the like.
The present inventors have attempted to drive the air cylinder by 5-port servo valves. However, controlling the pressurizing force by the 5-port servo valves separately requires pressure control valves. This has undesirably brought about upsizing and difficulty in cost-reduction.